In recent years, the development of TFTs using a transparent oxide semiconductor for an active layer is in progress.
U.S. Patent Application Publication No. US-2006-108636 A1 discloses a TFT technique using an amorphous transparent oxide semiconductor film (IGZO film), which includes In, Ga, Zn and O, as an active layer. The TFT, which is transparent and can be formed at a low temperature, is drawing attention because the TFT enables formation of a transparent TFT on a flexible substrate of, e.g., plastic.
FIG. 2 illustrates the structure of a typical bottom gate-type TFT using an oxide semiconductor. The typical bottom gate-type TFT is formed by depositing a gate electrode 2, a gate insulating layer 3, a semiconductor layer 4, source-drain electrodes 5 and a protective layer 6 on a substrate 1.
Upon a voltage being applied to the gate electrode, a channel is formed in a part of the semiconductor region at the inner portion of the semiconductor layer located above the gate electrode. However, no channel is formed in the majority of the semiconductor region located below the source-drain electrodes, where serves as a sort of resistor (which is also referred to as “parasitic resistance”). Accordingly, in order to enhance the current drive capability, it is necessary to lower the resistance of the semiconductor layer below the source-drain electrodes to provide improved electrical matching between the region where the source-drain electrodes are formed and the corresponding semiconductor region.
An object of the present invention is to provide an oxide semiconductor TFT that provides excellent electrical matching between a source-drain electrode and a semiconductor layer.